Compound vehicular transmissions having a main section connected in series with an auxiliary section are well known in the art. Typically, the main section is shifted directly with a shift lever connected to a shift bar housing or single shift shaft while the auxiliary section, whether a range section, splitter section, or combined range/splitter section, is shifted remotely using one or more valves and pistons. An auxiliary section ratio change may be indicated by the position of the shift lever, or may be initiated by a separate switch on the shift lever, such as a hi/lo range switch and/or a hi/lo split switch.
Since the range section typically utilizes synchronized jaw clutches, various systems have been developed to assure that a range shift is initiated and completed while the main section is in neutral to prevent undue wear and/or damage to the range section synchronized jaw clutches. Systems which prevent or delay initiation of a range section shift until the main section is in neutral are disclosed in U.S. Pat. Nos. 2,654,268; 3,138,965; 3,229,551; 4,060,005; 4,450,869; 4,793,378; and 4,974,474 the disclosures of which are hereby incorporated by reference. A valve assembly for protecting the range section synchronized jaw clutches in the event that the main section shift is completed before the range section shift is disclosed in U.S. Pat. No. 5,193,410, the disclosure of which is hereby incorporated by reference in its entirety.
U.S. Pat. No. 5,193,410 discloses a valve assembly which controls pressurizing and exhausting of a differential area piston to control the force applied to shift the range section based on the neutral/engaged condition of the main section. The valve assembly uses a valve which is spring biased to a first position for engaging a high speed auxiliary section ratio with a relatively high force. The spring bias is overcome by pressurized fluid to shift the valve to a second position for engaging a low speed auxiliary section ratio. However, if the fluid pressure decreases below a predetermined threshold for any reason, such as a system leak or compressor malfunction, the spring bias may shift the valve to the first position resulting in an unintended ratio change in the range section.